New micro-machining techniques are required to meet the growing demand for miniaturized products and processes. Abrasive waterjets have the potential to develop into an important micro-machining technique, but before this can happen new technologies are needed to generate and to control the flow of pressurised water flows carrying abrasive particles.
Micro-abrasive waterjets are formed by passing a pressurised suspension of abrasive particles in a fluid, generally water, through a ceramic or diamond cutting nozzle.
Abrasive suspensions can be provided pre-mixed, at the concentration required at the cutting nozzle, or alternatively abrasive particles can be metered from a bed of abrasive into a flow of fluid to a cutting nozzle.
Pre-mixed suspensions are normally formed by mixing abrasive particles and a suspending additive with water. A cartridge is filled with the suspension and is loaded into an abrasive storage vessel that forms part of the apparatus, or the suspension is caused to flow into an abrasive storage vessel. A pressurised source of water is then used to displace the abrasive suspension out of the abrasive storage vessel to a cutting nozzle. If sub micron abrasive particles or a viscous fluid is used, then a suspending additive may not be necessary. An abrasive storage vessel with a volume of one quarter of a liter contains sufficient suspension to cut for an hour with a 15 μm diameter nozzle operating with a water pressure of 700 bar.
When a micro-abrasive waterjet is to be fed with abrasive particles metered from an abrasive bed, the abrasive is first mixed with the fluid, usually but not necessarily water, and if needed, a Theological modifying additive. A cartridge is filled with the mixture and is loaded into an abrasive storage vessel or the mixture is caused to flow into an abrasive storage vessel. To carry out cutting about 10 percent or so of the flow from a pressurised source of fluid is diverted to the top of the abrasive storage vessel. The fluid flow into the abrasive storage vessel displaces a mixture of abrasive and fluid out of the outlet of the vessel, which mixes into the remaining 90 percent or so of the fluid that is flowing directly to the nozzle. A quarter liter abrasive storage vessel, containing a mixture with 70 percent abrasive by weight, can provide a suspension at a concentration of 10 percent abrasive to a 50 μm diameter nozzle for about one hour when cutting operations are carried out at 700 bar water pressure.
Cutting technologies using abrasive suspensions have been used in oil and gas well drilling and maintenance operations. Sand particles and/or particles of other materials are suspended in a water-based mud using bentonite and/or water soluble polymers, or in water using water-soluble polymers, and are pumped down a well to one or more relatively large cutting nozzles. More recently, U.S. Pat. No. 5,184,434 has described the use of similar water-soluble polymers in the generation of suspension abrasive waterjets for precision machining. For cutting operations with pre-mixed suspensions, an additive such as xanthan gum with shear-thinning characteristics is desired, so that it may hold abrasive particles in suspension when the suspension is not flowing, but not impede flow when cutting operations are in progress.
Oil well pumping equipment is large and robust and is capable of pumping abrasive suspensions. However, existing pumps for abrasive waterjet apparatus cannot handle abrasive suspensions in a satisfactory manner. An example of an apparatus that avoids pumping abrasive suspensions to generate abrasive waterjets is described in U.S. Pat. No. 5,184,434. It has valve arrangements to fill abrasive suspension storage vessels at low pressure and to discharge them at high pressure. The valves for such apparatus are required to open and close reliably with abrasive suspensions. However, valve technologies have not yet been available to build reliable valves for such apparatus.
International Patent Application WO 99/14015 (PCT/GB98/02627) describes apparatus suitable for producing micro abrasive waterjets.
The pressure differential imposed across settled beds of abrasive particles, with mean particle diameters greater than about 100 μm, causes water to percolate through the bed. Therefore, the mixture flowing out of the bed has a higher water content than is present in the bulk of the bed. Abrasive waterjets operating with settled beds of abrasive particles have relied on this water percolation for the bed to form and to aid in the flow of abrasive particles out of the beds. However, water percolation practically ceases with abrasive particle sizes needed for micro abrasive waterjets and this affects not only how beds can be formed, but also the time dependent rheological properties of abrasive beds and the structure of the beds during operation of abrasive waterjet apparatus.
Abrasive water mixtures of up to 70 percent by weight abrasive particles are used to form beds in apparatus to generate micro abrasive waterjets. Such mixtures exhibit complex, time dependent properties, such as thixotropy and hindered settling of particles. A bed may retain for several hours the characteristics of a freshly prepared mixture or may not reach a near fully settled state for many days. As particle sizes are reduced to micron and sub micron sizes, abrasive particle/water mixtures can begin to take on the properties of colloidal suspensions.
Polymer additives that are used to increase the viscosity of water are known to reduce the viscosity of mixtures with high ratios of abrasive particles to water. The additives affect the electrical charges of the particles and the interstitial water to allow easier movement between particles. Additives such as hydroxyethyl cellulose are known to prevent de-watering of abrasive particle/water mixtures by impeding the loss of water from abrasive beds.
Additives can be added to abrasive/water mixtures to provide benefits in operating abrasive waterjet apparatus. These benefits include:                a) Decreasing or increasing mixtures viscosity depending on the abrasive, water and additive concentrations, and on particle and additive properties;        b) Minimizing the-watering of the base of abrasive beds during cutting operations;        c) Aiding in the diffusion into abrasive beds of water entering the base of beds during pressurization of abrasive waterjet cutting apparatus and during abrasive on/off operations. This prevents the formation of vertical weakness in beds through which water can flow from the top to the bottom of a bed when only part of the bed has been discharged;        d) Maintaining desirable mixture characteristics for extended periods of time, particularly when abrasive is provided in cartridges that need a long shelf life; and e) Reducing the tendency for blockages to form in passages when conditions exist for abrasive particles to settle out, such as when the apparatus is not used fox an extended period of time and during upset conditions that cause high abrasive concentrations in passage.        
As there is a need to produce abrasive waterjets with a wide range of particle diameters and to control the jet formation and cutting operations it would be beneficial to provide apparatus which can operate with freshly prepared abrasive water mixtures, with abrasive/water mixtures that contain rheological modifying agents, and which can feed cutting nozzles from both suspensions and beds of abrasive particles.
Water compressibility is a major factor in the design and control of an abrasive waterjet apparatus. The compressed water volume in the abrasive storage vessel can be the equivalent of over 10 seconds of water flow through the apparatus. Precise control of cutting demands that this is vented away from the nozzle, usually by depressurizing the apparatus. When an abrasive waterjet apparatus is depressurized, this compressed water is violently expelled from the abrasive storage vessel through conduits to a vent valve. If the expelled water contains abrasive particles, the sealing capabilities of the valve seats of conventional valves can be destroyed in a single venting operation. There is thus a need for a valve that can handle highly abrasive flows.
To depressurize and depressurize an abrasive waterjet apparatus between the end of one cut and the start of a new cut may takes several seconds which represents lost machining time. It would be desirable to provide a valve in the flow passage to the cutting nozzle in order to stop the discharge from the nozzle without having to depressurize the apparatus. With a valve in the connection to the nozzle it is not necessary to cycle the pressure in the apparatus from a high to a low pressure in order to stop flow from the nozzle. This has beneficial effects in reducing fatigue loads on apparatus, improving pump and component reliability and reducing energy use.
Without a shut off valve before the cutting nozzle, abrasive is discharged through the cutting nozzle in a poorly controller manner during pressurization of abrasive waterjet apparatus. Poor control over abrasive flow has adverse effects on the way jets penetrate into work pieces and in particular can cause local widening of the cut width and cause jets to deviate.
In order to extend the capabilities of abrasive waterjet cutting apparatus to carry out percussion drilling, milling and marking requires cutting jets to be turned on and off many times per second. An effective way of achieving rapid on/off capabilities is to have an on/off valve in the connection to the cutting nozzle or for the cutting nozzle to be an integral part of an on/off valve.
Being able to start and stop the flow to an abrasive waterjet cutting nozzle by opening and closing a valve simplifies the control system for an abrasive waterjet apparatus and reduces the incidence of nozzle blockages.
Also, in the apparatus described in International Patent Application WO 99/14015 and in this application, there is described a means of replenishing the abrasive storage vessel with abrasive mixture from another vessel. This requires valves that operate reliably on abrasive/water mixtures.
As described above there are many reasons why the operation of abrasive waterjet apparatus would benefit from valves that could operate reliably on abrasive/water mixtures. However, suitable valves have not heretofore been known.
There are two basic types of mechanical valve mechanisms, both of which involve a port or aperture in a member, referred to as a seat, and a valve element. In one type of valve the element moves along the axis of the seat and in the other the element, or a second seat, moves transversely to the seat.
Valves that involve elements that move along the axis of a seat are not suitable for use with fluids containing highly abrasive particles because of the brittle mature of the ultra hard materials needed to resist erosion. Substantial forces have to be applied to achieve a seal between an axially moving valve element and a seat. When brittle materials are forced together to stop the flow through a valve, point contacts occur that create local high contact forces and these forces cause fracture of brittle materials.
Thus, valves for highly erosive conditions need a mechanism involving a valve element moving more or less at right angles to a seat in such a way that abrasive particles cannot get between contacting surfaces. Ball valves and rotary disc type valves, with spring loaded elements to stop abrasive particles getting between contacting surfaces, have been developed for systems that operate with fluids that contain highly erosive particles.
However, such valves have limitations as regards apparatus to generate micro abrasive waterjets because:                a) Valve elements and seats cannot be easily fabricated from ultra hard materials to withstand wear if the valves are to be closed or opened under the high pressures in abrasive waterjet cutting apparatus;        b) The small size of the valve elements needed for micro abrasive waterjet apparatus makes it impractical to provide robust drive mechanisms that penetrate through pressure containments to actuate valve elements;        c) Sealing of valve element drive mechanisms, where they do pass through the pressure containment, is very difficult in the presence of the fine abrasive particles used in micro abrasive waterjet cutting; and        d) The valves have flow passages that contain spaces where abrasive particles can accumulate and subsequently be released, when the sudden release of accumulated abrasive can cause cutting nozzles on abrasive waterjet apparatus to block.        
It is therefore another object of this invention to provide two mating valve seats that slide relative to one another so that apertures in the seat can be aligned for flow to pass through the valve. Flow may be stopped by sliding the seats relative one to the other until the apertures no longer provide a flow path.
Although the valves will operate in the presence of abrasive suspensions, it is desirable that the amount of abrasive present during opening and closing of such valves is minimised. A means of momentarily stopping abrasive flow, in order that valves in the connection to the cutting nozzle may be operated in the presence of water alone, is described in International Patent Application WO 99/14015, and is incorporated into certain of the embodiments of the present invention.
Plunger pumps are conventionally used to power abrasive waterjet apparatus. Such pumps suffer from delivery pressure ripple. Pressure ripple can be minimised by synchronizing the motion of a plurality of pump plungers, as described in International Patent Application WO 99/14015, but some pressure ripple will always remain. Abrasive waterjet apparatus can function satisfactorily in cutting mode with a significant level of pressure ripple but problems arise when the abrasive flow out of an abrasive storage vessel is turned off by stopping the water flow into the top of the vessel. Water compressibility causes the abrasive storage vessel to act as a fluid accumulator, so a drop in pump delivery pressure, or an increase in pressure losses due to operating a valve to turn the abrasive off, causes abrasive to continue to flow out of the abrasive storage vessel.
There is thus a requirement for an apparatus and a method of operation thereof which may control or eliminate the adverse effects of such pressure variations. In the apparatus described, the pump delivery pressure is increased in a controlled manner when the abrasive off valve is operated. The pressure increase is greater than the sum of pressure variations caused by the pump and the pressure drop caused by operating the abrasive off valve, thereby ensuring that abrasive flow out of the abrasive storage vessel is stopped when the abrasive off valve is operated.
According to a first aspect of the present invention, there is provided a valve adapted to control a flow of abrasive particles suspended in a pressurised carrier fluid, comprising at least two apertured valve seat means each having a contact face in contact with a corresponding opposing contact face of another of said at least two apertured valve seat means and being translationally slideable in contact therewith and with respect thereto between a first position in which the apertures of each valve seat means are aligned so that fluid may pass through said apertures, and a second position wherein the aperture in one valve seat means is blocked by the contact face of another to stop flow through the valve, wherein the valve seat means each comprise an outer layer of material with a hardness, as measured on the Mohs scale, of at least 9.
Preferably there are provided two valve seat means, one being translationally slideable in contact with the other and with respect thereto.
Alternatively, there are provided three valve seat means, a median one of which being translationally slideable in contact with the outer ones and with respect thereto.
Advantageously, each of the valve seat means comprises diamond.
At least some of the valve seat means may comprise a composite diamond/ceramic material.
In this case, a median one of the valve seat means may comprise two layers of such composite material, with their ceramic faces brazed or otherwise joined together.
The valve may be provided with means to urge said valve seat means together.
The valve may comprise spring means adapted to urge the valve seat means one towards the other.
Additionally or alternatively, the means to urge the valve seat means towards one another may comprise the pressure of the carrier fluid exerted on one of the valve seat means.
In this case, the flow of abrasive particles and carrier fluid may pass to a seat means through a tube adapted to allow sliding movement of the seat means and to transmit thereto a force urging the seat means together.
The tube should withstand any buckling force.
The valve may be adapted to operate at a pressure of at least 1000 bar (100 MPa).
The abrasive particles may have a hardness of at least 6 Mohs.
The valve may be provided with slide means, to which one of the valve seat means is mounted, adapted to be moveable translationally by external actuating means, thereby causing said one valve seat means to move between said first and said second positions.
Advantageously, said external actuating means are pneumatic actuating means.
Optionally, said slide means may be configured to act as a piston means within a double-ended cylinder means provided with inlet means at each end for compressed actuating air.
Turning means may be provided to rotate at least one of said valve seat means and/or its slide means in relation to the other.
The valve may have a single inlet means leading to the aperture in one valve seat means and a single outlet means leading from the aperture in the other valve seat means, the valve containing as a result no dead spaces where abrasive particles may accumulate.
One or each valve seat means may have a contact face grooved to allow replenishment of a lubricating molecular water layer between the contact faces.
Additionally or alternatively, one or each valve seat means may comprise porous polycrystalline diamond so that a flow of water may penetrate the or each valve seat means sufficient to lubricate the contact surface between the valve seat means.
Advantageously, there is provided a container assembly adapted to contain supply of abrasive particles for use in an abrasive fluid jet machining apparatus, said assembly comprising a container for said abrasive particles closeable sealably by means of a cap, said cap comprising an inlet means connected to a riser tube within said body, each of such restricted bore as substantially to prevent liquid flow therethrough, except under an imposed pressure differential, and an outlet means, the bore of which comprises such a restriction as substantially to prevent flow therethrough, except under an imposed pressure differential.
Hence, the inlet means and outlet means are adapted to resist liquid flow out of the container assembly in the absence of sealing means.
The cap may comprise a substantially circular end face and said outlet means is disposed substantially centrally thereof.
Advantageously, said inlet means is disposed substantially flush to an end face of said cap.
The riser tube may extend from an inner face of said cap to a point adjacent but not in contact with a remote end of the container.
The container may contain a supply of abrasive particles suspended in a carrier fluid.
Alternatively, the container may contain a supply of abrasive particles immersed in a carrier fluid to form a bed of abrasive particles, adapted initially to occupy approximately 90% of the body of the container.
Preferably, an upper end of said riser tube is disposed above said bed when the container assembly is oriented with the cap at a lower end thereof.
The preferred carrier fluid is water.
In this case, the bed of abrasive particles additionally comprises a water-retention aid.
Advantageously, said abrasive particles comprise particles of garnet, olivine or aluminum oxide.
Optionally, said abrasive particles may have a mean particle diameter of between 10% and 50% of the diameter of the nozzle. The mean particle diameter may be less than 10 μm.
According to a second aspect of the present invention, there is provided an apparatus for machining a workpiece, comprising pressurizing means, a storage vessel for a supply of abrasive particles, a nozzle, and a valve as described above adjacently upstream of the nozzle, adapted to interrupt flow through the nozzle.
The pressurizing means may further comprise means momentarily to increase the pressure at a point between the nozzle and the storage vessel prior to actuation of the valve to interrupt flow through the nozzle.
The pressure at said point may be raised to a level exceeding that present in the storage vessel.
The apparatus may include valve means openable to cause an increased proportion of the fluid to flow from the pressurizing means directly to the point.
The apparatus may comprise means to control the pressurizing means to vary the delivery pressure.